I find it creepy that Gentiles enforce Sabbath-keeping and celebrating national Jewish holidays as requisites to salvation, and the SDA included.
The reason the SDA does this is the belief in a doctrine called Anglo-Israelism. This belief teaches that white Americans are descendants of the "lost" tribes of Israel. As such they believe that they should be bound to the tenets of the Mosaic Law because it is the constitution of the Tribes of Israel.
Among the other groups that share this belief are the one-time sister religion of the JWs known as the Worldwide Church of God under Armstrong (who also predicted that the world would end in 1975, by the way), and the Mormons (though the LDS does not keep the Jewish Sabbath, per se).
Now you can actually swab the inside of my cheek and find DNA specific hereditary markers of a Sephardic Jew of Semite origins, but these deluded people cannot do the same (I've had some of these tell me that DNA evidence is irrelevant or that G-d changes their blood to be Jewish in a spiritual way or perhaps the Devil is tampering with DNA results to hide the truth). My thoughts is you can take any day off at any time of the week and get the same effect as a good Jewish Sabbath.
Besides, why observe the Sabbath like a Jew and celebrate Jewish feasts like a Jew and dress like a Jew, etc., but belong to a different religion? Just convert if you like it that much. Otherwise a little borrowing here and there from this and that doesn't require you join any group or organization of any kind.
In my opinion you are better off being true to yourself without denominational ties for now. Take in what's around you, learn from life, and if you end up adopting a certain philosophy or ritual along the way, let it be something that adds to your freedom, not takes away from it.